The use of composite materials in the aerospace industry has considerably increased since the 1970s. Good mechanical resistance of said materials and their low weight have increased their use in different structural elements of aircraft, except in those areas subjected to high thermal loads.
The object of the invention is to provide a composite material that increases noise attenuation (for example, when it is used as structural material in the airframe of an aircraft), thereby improving passenger flight comfort. It also seeks to increase impact resistance as a structural material of the aircraft. Good electric conductivity is also sought as it allows a lightning bolt to easily exit the aircraft after striking it, for example. Said material comprises a structure which can be continuous for all its layers, made up of a base formed by a matrix and the corresponding fiber, at least one sheet of viscoelastic material, and at least one layer of impact-protection material.
Various forms of the combined use of composite and viscoelastic material are known in the state of the art, namely:                technology known as embedded damping, consisting of using a sheet of viscoelastic material embedded in a composite material for absorbing and/or dissipating energy in intermediate-frequency vibrations (from 100 to 500 Hz);        technology referred to as add-on damping, based on the use of an element connected to a layer of viscoelastic material that is adhered to a structural element for absorbing and/or dissipating energy in intermediate-frequency vibrations (from 100 to 500 Hz).        
Various patent documents with a state of the art related to the object of the present invention are also known. International patent application WO 2008/147754 A1 describes a hybrid composite material for damping sounds and vibrations in aircraft. This material has various layers, a viscoelastic layer among them, which can have metal fibers to increase the damping effect.
International patent applications WO 2010/077849 A1, WO 2010/079322 A1 and WO 2008/115301 A2 relate to multilayer composite materials, including at least one viscoelastic layer or the introduction of elements with viscoelastic properties.
United States patent application US 2010/0126796 A1 relates to a multilayer composite material with damping properties, having a viscoelastic layer between other metallic layers.
Patent document EP 1,500,494 B1 discloses a composite material the structure of which has damping layers of viscoelastic material, and it relates to the manufacturing process thereof.
Patent document U.S. Pat. No. 6,114,050 A describes a titanium-polymer hybrid laminate that combines layers of composite material; in a preferred embodiment all the layers of composite material have the same orientation. The material is particularly applicable for supersonic civilian aircraft.
However, there is no known material in the state of the art like the material described herein in which all the aspects of this invention, such as noise attenuation, electric conductivity and impact resistance, are efficiently combined. Impact resistance must be understood as low, medium and high energy impact resistances. Low energy impacts are understood as those that are less than 50 J, representative of a tool falling on the material. Medium energy impacts are those comprised between 50 J and 3 kJ; typical examples are impacts of birds during flight. Finally, high energy impacts are those impacts that exceed 3 kJ, representative of the impact of ice fragments expelled by open rotor or propeller engine blades on the fuselage.